Begin by preparing your vegetables. Shred 1/2 head of cabbage to your preferred thickness—finer shreds create a more delicate texture, while chunkier pieces offer more bite.
Shred 1 medium carrot using the same method, then dice 1/2 cup of onion into small, uniform pieces. Set these vegetables aside in a large mixing bowl.
In a separate bowl, combine your dressing ingredients: 2 cups mayonnaise, 2 tablespoons white vinegar, 2 teaspoons sugar, and 2 teaspoons celery seeds.
Whisk these together until well blended and smooth. Pour the dressing over the cabbage, carrot, and onion mixture, then toss everything together until all the vegetables are evenly coated and incorporated with the creamy dressing.
Transfer your coleslaw to the refrigerator and allow it to marinate for at least an hour, though leaving it overnight or for up to a day will develop deeper flavors and help the vegetables become more tender.
The longer marinating time also allows the vegetables to release their natural juices, which blend beautifully with the dressing. Once properly chilled and marinated, your homemade coleslaw is ready to serve alongside your favorite BBQ, sandwiches, or main dishes.